1. Field of the Invention
The invention relates to telephone signalling. More particularly, the invention relates to methods and apparatus for intercepting DTMF dialing signals and redialing the intercepted signals with additional DTMF signals appended to or embedded in the intercepted signals.
2. State of the Art
Telephone technology has evolved in the twentieth century from a relatively simple concept to a vastly complex system. Some of the most significant changes have been in telephone switching techniques. Early telephones were switched manually. Individual telephone operators provided interconnections between subscribers by plugging a wire connected to the calling telephone set into a switchboard in order to connect it to a wire connected to the called telephone set. The first automatic telephone switching exchange used step-by-step switches with 10xc3x9710 switching fields to connect two digits of a dialed number. Later, crossbar switches were developed and are still in use in many parts of the world today. A crossbar switch includes a rectangular array of electromagnets each of which activate mechanical linkages to close a set of contacts. During the nineteen sixties and seventies, many telephone companies replaced the mechanical crossbar switches with solid state electronic switches which have a much higher capacity in the number of subscribers served by a single switch. Electronic switches provide many advantages over electromechanical switches, one of which is that they enable tone dialing.
The early electromechanical switches responded to DC xe2x80x9cdial pulsesxe2x80x9d which are still used by some telephones today. The dial pulses are generated by a rotary mechanical switch in a subscriber""s telephone which results in alternating xe2x80x9con-hookxe2x80x9d and xe2x80x9coff-hookxe2x80x9d signals. More particularly, dial pulses occur at a nominal rate of ten hertz with a break period of 61% (or about 60 milliseconds) and a closed circuit period of approximately 40 milliseconds. Digits dialed are indicated by the number of break periods. For example, the digit three is indicated by three openings of the dial contacts. The higher the cardinality of the digit, the longer it takes to dial the digit. In other words, it takes 1 second to dial the digit zero, nine tenths of a second to dial the digit nine, etc. As mentioned above, the advent of electronic exchange switching has enabled the implementation of tone dialing. The international standard for tone dialing is known as dual-tone multi-frequency (DTMF) signalling. The DTMF system uses eight tones having very specific frequencies. Signalling is effected by transmitting tones in pairs. The DTMF tones are generated in the subscriber""s telephone when buttons on the phone are pressed. The DTMF standard supports sixteen DTMF signals but typically only twelve (digits 0-9 and symbols * and #) are used. The additional signals (usually referred to as A-D) are used in signalling between switches). Table 1 illustrates the assignment of tones to the standard twelve dialing signals.
As seen in Table 1, four xe2x80x9crow frequenciesxe2x80x9d 697 Hz, 770 Hz, 852 Hz, 941 Hz, and four xe2x80x9ccolumn frequenciesxe2x80x9d 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz form a sixteen cell matrix within which DTMF signals are defined. For example, the digit 1 is signalled by transmitting a 697 Hz tone and a 1209 Hz tone simultaneously. The digit 9 is signalled by transmitting a 852 Hz tone and a 1477 Hz tone simultaneously. The minimum duration of each DTMF signal is approximately 40 milliseconds and the minimum time between DTMF signals is also approximately 40 milliseconds. The minimum time it takes to xe2x80x9cdialxe2x80x9d a digit with DTMF signalling is independent of the cardinality of the digit dialed.
The DTMF frequencies were chosen so that all of the tones would be easily transmitted within the limited bandwidth of a voice telephone circuit (about 300-3,000 Hz) and so that no tone is an harmonic of another tone. The elimination of harmonics was intended to enable DTMF tone detectors to correctly discriminate between DTMF signals and melodious human speech. Human speech is rich in harmonic tones and that is one reason why computer generated speech rarely sounds like true human speech. According to the DTMF detection system, if an harmonic of a DTMF tone is detected together with the tone, the tone is not recognized as a DTMF signal component.
One of the primary advantages of DTMF dialing is that digits can be dialed much faster and more accurately with push buttons using DTMF tones than with a rotary dial. For example, a ten digit telephone number can be dialed (by a device) using DTMF in 760 milliseconds. Using pulse dialing, a ten digit telephone number will take, on average, about five seconds to dial. The importance of rapid dialing is increasing because of political and technological changes in the public telephone network. Technologically, the seven digit telephone number is becoming a thing of the past since the number of subscriber lines installed in some urban areas exceeds 9,999,999 (the actual number of seven digit telephone numbers is less than ten million because of certain dialing conventions and reserved numbers). Thus, it is becoming common in many urban areas that even xe2x80x9clocal callsxe2x80x9d require the dialing of an area code. Politically, when the telephone monopoly in the U.S. ended, competing service providers required the dialing of xe2x80x9caccess codesxe2x80x9d in order to select among different service providers at the time a call is made. Originally, the access codes consisted of five digits in the form 1-0-x-x-x, where the digits x-x-x identified a particular service provider. Recently, the format of access codes has been changed to seven digits in the form 1-0-x-x-x-x-x in order to accommodate more competing service providers. Thus, the number of digits presently required to make a call with a selected service provider is eighteen (seven for the access code, seven for the phone number, and four for 1+area code). International calls require even more digits. Many service providers are finding that consumers are unwilling to dial so many digits (even with a push button phone). The competing service providers are therefore losing business to the default service provider initially chosen by the consumer. In addition, consumers are reluctant to choose a different service provider on a call by call basis because of the need to dial many more digits.
In view of the foregoing, it is apparent that consumers need some relief from the number of digits needed to be dialed when using a service provider which is selected by the dialing of an access code. One possible solution is to program a telephone having a memory with the seven digit access code and assign that number to a xe2x80x9cspeed dialxe2x80x9d button. However, not all telephones have memory or speed dialing features. In addition, the typical household has many xe2x80x9cextensionxe2x80x9d telephones and each telephone would need to be programmed with the speed dialing access code.
It is therefore an object of the invention to provide methods and apparatus by which a telephone user can access a telephone service provider without dialing the access code normally required to access the provider.
It is also an object of the invention to provide methods and apparatus which automatically insert a service provider access code when a long distance number is dialed.
It is another object of the invention to provide methods and apparatus by which all extension telephones on a single subscriber line can automatically access a telephone service provider without dialing the access code normally required to access the provider and without programming each of the telephone extensions.
It is a further object of the invention to provide methods and apparatus for intercepting DTMF dialing signals and redialing the intercepted signals with additional DTMF signals appended to or embedded in the intercepted signals.
Another object of the invention is to provide a single apparatus for intercepting DTMF dialing signals and redialing the intercepted signals with additional DTMF signals appended to or embedded in the intercepted signals, which single apparatus intercepts signals from all extension phones coupled to a single subscriber line.
In accord with these objects which will be discussed in detail below, the methods of the present invention include (a) monitoring a subscriber line for the presence of DTMF tones, (b) intercepting DTMF tones by transmitting a set of DTMF harmonics onto the subscriber line to prevent a central office (CO) from detecting a DTMF signal, (c) detecting the intercepted DTMF tones and recording them or the digits they represent, (d) analyzing the intercepted digits to determine whether a long distance number has been dialed, and (e) if a long distance call has been dialed, transmitting onto the subscriber line a DTMF signal which includes the intercepted DTMF signal plus a service provider access code DTMF signal inserted before it, and if a local call has been dialed, transmitting the intercepted signal without the access code.
An apparatus according to the present invention includes (i) a modified DTMF sensor, (ii) a harmonics tone generator, (iii) a temporary memory for storing the intercepted DTMF tones or the digits the intercepted tones represent, (iv) a permanent memory for storing a service provider access code or the DTMF signal representing the access code, (v) logic for analyzing the intercepted digits, and (vi) a dialer for dialing the intercepted digits and the access code.
According to presently preferred embodiments of the invention, the apparatus includes a voltage sensor which detects a drop in voltage whenever a telephone set connected to the same subscriber line as the apparatus goes off-hook. The voltage detector turns the apparatus xe2x80x9cONxe2x80x9d when an off-hook condition is detected. The presently preferred apparatus also includes an energy sensor which rapidly detects the presence of any fixed frequency tone and triggers the harmonics tone generator. The harmonics tone generator preferably generates the second harmonic of each of the xe2x80x9crow frequenciesxe2x80x9d of the DTMF standard. Thus, all DTMF tones are intercepted before they are recognized. The modified DTMF sensor includes means for filtering out the four harmonics generated by the harmonics tone generator. This arrangement assures that any DTMF tone generated by a phone connected to the subscriber line is intercepted and recognized within 40 milliseconds, the minimum time it takes a CO to recognize the DTMF tone. According to the preferred embodiment, as the tones are recognized, the digits the tones represent are stored in a temporary memory for analysis by control logic. The control logic determines what type of number has been dialed (e.g. local, long distance, international) and determines whether an access code should be inserted before the dialed number. The logic then causes the dialer to dial the number and, if appropriate, to dial the access number first. In addition, the logic turns the apparatus xe2x80x9cOFFxe2x80x9d after the dialer has finished dialing. This prevents the energy sensor from triggering the harmonics generator during voice traffic. The apparatus will thus remain idle until a new off-hook condition is recognized.
According to one embodiment of the invention, an international call (i.e. a call which does not have a fixed number of digits) is recognized by the prefix xe2x80x9c011xe2x80x9d but the end of dialing is not automatically determined. After an international number is dialed, the person making the call must dial xe2x80x9c#xe2x80x9d to signal the apparatus to dial the number with the inserted access code. According to another embodiment, the apparatus is provided with a data base which analyzes international numbers to determine when dialing is completed.
According to still another embodiment of the invention, the apparatus is provided with a database of several access numbers for different service providers. The user may manually select from a menu of service providers by dialing #1, #2, etc. after dialing a number. According to yet another embodiment, the logic of the apparatus is programmed to automatically use the most economical service provider based on time of day, number dialed, etc.